I. Field of the Invention
The present invention relates to digital signal processing. More particularly, the present invention relates to a novel and improved method and apparatus for decoding a frame of digital data that contains an outer error detecting code, e.g., redundant check bits, or any other side information which is known about the data.
II. Description of the Related Art
As wireless communication technology has advanced, users of those systems have come to demand higher transmission quality is in the services provided. One means of providing higher transmission quality to provide that service using digital transmission techniques. The use of code division multiple access (CDMA) modulation is one of several techniques for providing digital wireless transmission. Other methods of digital wireless transmission include time division multiple access (TDMA) and frequency division multiple access (FDMA).
However, the spread spectrum modulation technique of CDMA has significant advantages over other digital modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein. The method for providing digital wireless communications using CDMA was standardized by the Telecommunications Industry Association (TIA) in TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System (hereafter IS-95).
Because each user's transmission contributes incrementally to the noise to other user's in a CDMA communication system, the capacity of a CDMA system is maximized by having each user transmit only as much data as necessary. A very effective means of reducing the user's burden on capacity without reducing the quality of service to that user is by means of variable rate data transmission, where data rate of transmissions by the user varies with time. A method for providing variable rate encoding of speech data is described in detail in U.S. Pat. No. 5,414,796, entitled "VARIABLE RATE VOCODER", which is assigned to the assignee of the present invention and is incorporated by reference herein.
In order protect against channel induced errors, digital error correction techniques are employed in wireless communication systems. One method for detecting and correcting for channel induced errors is through the employment of trellis decoding. A Viterbi decoder is a specific type of trellis decoder which has shown significant advantages over other error correction systems. A method and apparatus for performing Viterbi decoding of variable rate data are described in detail in copending U.S. patent application Ser. No. 08/126,477, filed Sep. 24, 1993, entitled "MULTIRATE SERIAL VITERBI DECODER FOR CDMA SYSTEM APPLICATIONS", which is assigned to the assignee of the present invention and incorporated by reference herein.